(a) Field of the Invention
This invention relates to fabrics as used in the dryer sections of paper making machines.
(b) Description of Prior Art
In the manufacture of paper on a Fourdrinier paper making machine, for example, an aqueous suspension of cellulose fibres, comprising one part or less fibres in 99 parts or more of water by weight, is flowed on to an endless rotating forming screen woven of metal or synthetic filaments. As this belt, or forming fabric or "wire", as it is called, passes over water extraction devices such as table rolls, drainage foils and suction boxes, the water content of the suspension supported on the fabric is reduced to about 80 to 85 percent.
The thin web of fibres, now self supporting, is removed from the forming fabric and passes to a series of one or more press sections where it is deposited on other endless belts of relatively thick fabric, one or both surfaces of which may be composed of a needled bat of synthetic or natural fibres. These belts, called wet felts carry the web through the nips of press rolls where more of the water remaining in the web is squeezed into the absorbent felts until the water content is lowered to about 65% at which point it is not generally practical to attempt further water removal by direct extraction such as with pressure or vacuum.
The web of paper is then passed to the dryer section of the machine where the remainder of the water is removed by an evaporation process accelerated by the application of heat. The dryer section consists of a number of large, hollow cast iron or steel cylinders over which the paper web passes in a serpentine fashion. The cylinders are rotated synchronously to facilitate the passage of the web. Heat is supplied by steam condensing inside each cylinder and the web is held in intimate contact with portions of the heated surfaces of the dryer cylinders by the dryer fabrics.
To provide sufficient drying capacity a newsprint dryer section, for example, may consist of about 50 dryer cylinders each about 5 feet in diameter and set up in an upper and lower tier in four or five individual subsections.
In order to appreciate the magnitude of the dryer section of a modern paper making machine, the overall size may be about 200 feet long, up to 40 feet wide and up to 40 feet high. The paper web may pass through the dryer section at speeds up to 3000 feet per minute so that any part of the web may only remain in the dryer section for as little as 15 seconds during which time the web will be reduced to a normally dry sheet of paper.
The dryer fabrics serve to hold the paper web against the heated surfaces of the rotating dryer cylinders to promote more effective heat transfer to the web by partially eliminating a heat insulating layer of air which adheres to the surface of the cylinders. The drier fabrics also serve to prevent the paper web from wrinkling.
In the conventional dryer section there is an upper and a lower dryer fabric. The upper fabric wraps around and holds the paper web against the upper peripheries of the upper dryer cylinders while the lower fabric wraps around and holds the paper web against the lower peripheries of the lower dryer cylinders. The fabrics are guided by intermediate fabric rolls placed between the cylinders.
Dryer fabrics operate in a particularly adverse environment in which they are alternately exposed to hot and wet and hot and dry conditions. They must be flexible in the machine direction so that they can bend around the felt rolls easily. They must have good dimensional stability and durability under the conditions of tension, temperature and humidity which prevail in the dryer section of a paper machine. Generally, dryer fabrics are woven from either natural or synthetic yarns to form a relatively bulky fabric that will have good absorbent characteristics and high porosity to enhance removal of moisture from the web of paper. To attain these results the yarns are woven closely together and sometimes in several plies to form a comparatively impermeable fabric. To decrease permeability further sometimes bulky staple fibre yarns, some containing asbestos, are woven in. These fabrics thus exhibit an undesirable tendency to hold sufficient water to rewet the sheet. They also become increasingly difficult to clean of various foreign substances such as sizing agents, clay-like fillers and resins, gums, waxes and pitch and the fabric becomes plugged up so that it has to be cleaned frequently or repaced.
Dryer fabrics are usually woven with approximately 100% warp fill, as shown in the drawings of this application and as is well known to those skilled in the art. Warp fill is defined as the amount of warp in a given space relative to the total space considered. Warp fill can be over 100% when there are more warp strands jammed into the available space than the space can dimensionally accommodate in a single plane. Fabrics having a nominal warp fill of approximately 100% will generally have an actual calculated warp fill of from 80% to 125% as is the fabric of this invention. Values over 100% are brought about by crowding and lateral undulation of the warp strands.
Permeability is an important characteristic of a dryer fabric and is a measure of its air passage capability. A low permeability fabric will resist the passage of air and tend to absorb vapour whereas a high permeability fabric will allow free passage of air and vapour.
As indicated previously dryer fabrics were conventionally made from cotton or wool and sometimes contained asbestos fibres. With the development of synthetic yarn materials the conventional fabrics are gradually being replaced by fabrics containing synthetic yarns. These may be woven in simple or in very complex weaves in two or three plies or more of either relatively large diameter monofilament yarn or of multifilament yarns spun from many small diameter filaments.
Of the new synthetic yarns monofilaments are preferred because the resultant fabric has increased running life, is easy to clean, does not shed fibre and does not carry excessive moisture. During the part of the cycle when the fabric is in contact with the sheet over a dryer cylinder, low moisture content and high permeability enhance transfer of heat to the web. Also, the high permeability of the fabric can have a beneficial effect on ventilation of the dryer pockets, producing a more even moisture profile in the web. However, the high permeability of fabrics made from all-monofilament yarns in some cases is a disadvantage as it causes excessive air movement in dryer pockets which results in sheet flutter. This problem increases with machine speed and a point is soon reached when the flutter, particularly in the first and second dryer sections where the web is wet and weak, is violent enough to cause it to break.
The effect of fabric permeability on dryer pocket ventilation and sheet flutter has been described by Race, Wheeldon, et al (Tappi, July 1968 Vol. 51 No. 7) and they have shown that air movement in dryer pockets is influenced by permeability rather than by the surface roughness of the fabric as was previously supposed. Air movement in dryer pockets is induced by the fact that a moving fabric carries with it layers of air. At the surface of the fabric the velocity of the air layer is the same as that of the fabric and as the distance from the surface of the fabric increases the velocity of the air decreases. When the fabric wraps around a roll, the layer of air on the inside is trapped in the nip between the roll and the fabric and, if the fabric is sufficiently permeable, the air from the inside is pumped through, joins the air stream on the outside of the fabric and the combined velocity of the two streams is greater than the speed of the fabric. As the fabric passes around the roll the layers of air on the outside tend to be thrown outward by centrifugal force generating tangential air movement. This results in a large mass of air moving laterally out of the pockets when high permeability fabrics are used on high speed machines.
The Race, Wheeldon et al experiments show that as fabric speed increases, the air which is pumped through the fabric by the felt rolls of the dryer increases in velocity, particularly at speeds above 1500 r.p.m. They also show that as fabric permeability is reduced, the amount of air pumped into the dryer pockets is correspondingly reduced. Thus at low speeds a dryer fabric with high permeability can be tolerated and, in fact, is useful in achieving high drying rates, but at high speeds, particularly in the first or second dryer sections, it is necessary to have low permeability fabrics in the range of 50 to 200 cu.ft./min./sq.ft. Thus on high speed machines it is often not practical to take advantage of the easy to clean characteristic of monofilament fabrics because of their inherent high permeability.
"Permeability" is usually expressed by the number of cubic feet of air per minute passing through a square foot of the fabric when the pressure drop across it is 0.5 inches of water. One instrument used to measure air permeability is a Frazier Air Permeometer.
In this instrument air is drawn by a variable speed run through a 1 square inch section of fabric to be tested then through upper and lower chambers joined by one of a set of replaceable orifices calibrated for measuring volume by pressure differential. The speed of the fan is increased until the upper chamber reaches a vacuum of 0.5 inches of water as indicated on a manometer. The vacuum, in inches of water, in the lower chamber is then read off another interconnected monometer and this value is applied to a reference graph to convert the reading to cubic feet of air per minute per square foot of fabric.
While in the conventional dryer system, the problem of sheet flutter may be overcome by using a dryer fabric having low permeability, another method of alleviating this problem is known as the single fabric dryer system. In this method, a single dryer fabric is used to guide the web of paper in serpentine fashion through the dryer sections of the paper machine. The paper, for example, is introduced under the fabric at the first upper cylinder and passes substantially in contact with the fabric all through a dryer section so that it lies between the fabric and the cylinders in the upper tier and outside the fabric around the cylinders in the lower tier.
The main advantage of the single fabric dryer system is that the web of paper is partially supported by the fabric as it passes between the tiers of dryer cylinders and sheet flutter is thereby reduced or may be entirely eliminated.
Other important advantages of the single fabric dryer system include reduction of dryer fabric costs and elimination of felt rolls and one set of stretch and guide rolls which are no longer required. Also, since the lower tier of cylinders is not encumbered by a separate lower dryer felt, the waste paper from paper breaks, or "broke" as it is called, may be removed more easily.
A disadvantage of the single fabric system is that when it is applied to existing dryer sections in which all the dryer cylinders are the same size and are driven at the same rotational speed by an interconnected set of gears, the conventional monofilament fabric having a high modulus of elasticity, is quite inextensible and will try to force the upper cylinders, which have a larger effective diameter due to the layer of paper, to turn at a lower rotational speed. This braking action of the cylinders by force tending to stretch the fabric, produces considerable stress on the drive train and even when the web of paper is fairly thin, the stress has been sufficient to cause abnormal wear of the gear teeth and bearings and in some cases structural failure.
The stretch of the fabric, called fabric draw, caused by the difference in fabric path lengths over the cylinders is within the elastic range of the fabric and is proportional to the thickness of the web of paper. The stress, expressed in terms of torque, on the dryer cylinder gears, is proportional to the product of the paper thickness and the modulus of elasticity of the fabric. As a practical example, in a single fabric dryer section where the paper web is only 0.012 inches thick the calculated torque developed at the drive gear of an upper cylinder will amount to 3000 ft.-lbs. From this it will be apparent that the problem of gear wear and structural failure will be significantly alleviated by using a fabric having a lower modulus of elasticity so that it stretches more easily and can absorb the stress developed by differentials in dryer cylinder diameter due to paper thickness.
While the above example illustrates the degree of stress that can be developed by a relatively thin web of paper, it will be appreciated that differences in dryer cylinder diameters caused by wear or by thermal expansion due to temperature differentials may also have destructive effects which can be alleviated by using a dryer fabric having a lower modulus of elasticity.
The stress problem can be overcome in those cases where it is possible to disconnect the upper gear train from the lower gear train so that either the upper or the lower cylinders only are driven. In such cases the cylinders which are disconnected are rotated by the dryer fabric and it doesn't matter if they rotate at a different speed. There are some installations, however, in which it is not possible to disconnect some of the drive gears and it is in these cases where a fabric having low modulus of elasticity will be used to advantage.
A further disadvantage of the single fabric dryer system arises because of the relative thickness of a conventional fabric. For example, when the wet web of paper passes from an upper dryer cylinder where it lies under the fabric, to a lower dryer cylinder where it lies over the fabric, it is stretched due to the difference in diameters. This stretch, or paper draw, is proportional to the thickness of the fabric. Since it is easily extensible the wet web of paper will accommodate to the draw. However, as it progresses from a lower dryer cylinder to an upper dryer cylinder a negative draw is created and because the wet web of paper is non-elastic it separates from the fabric and billows out so that it can fold or overlap on itself before passing under the fabric at the upper dryer cylinder, thus nullifying the effect of the support of the fabric. It will be apparent therefore that it is advantageous to use the thinnest possible dryer fabric in the single fabric system.